1. Field of the Invention
This invention relates to a lead-free solder paste which is suitable for reflow soldering of chip components and which can prevent the occurrence of tombstoning during reflow soldering.
2. Description of the Related Art
Among soldering methods commonly in use today, the reflow soldering method is particularly suitable for bonding extremely small electronic components to printed circuit boards. In reflow soldering, a solder paste containing a fine solder powder and a flux is applied to selected areas of a printed circuit board by screen printing through a metal mask or silk screen. Electronic devices are then mounted on the applied solder paste and temporarily held in place on the printed circuit board by the stickiness of the solder paste. The printed circuit board and the electronic devices are then heated, usually in a furnace, to melt the solder powder in the paste. The molten paste is then cooled to solidify the molten solder, thereby forming soldered joints between the electronic devices and the circuit board.
The reflow soldering method enables minute electronic devices to be precisely positioned on a printed circuit board, and there is no undesired bridging of solder between adjoining electronic devices, so it can produce printed circuit boards of high accuracy and reliability. This method also has excellent productivity.
However, one problem which has been encountered when soldering minute electronic components to printed circuit boards by reflow soldering is the phenomenon of tombstoning (also referred to as the Manhattan phenomenon). In this phenomenon, one end of an electronic device becomes detached from a printed circuit board during reflow while its opposite end remains bonded to the circuit board, as a result of which the one end rises and the electronic device assumes a more or less vertical orientation. Tombstoning is caused by a difference in the time at which solder paste is melted at opposite ends of an electronic device during heating in a reflow furnace. The surface tension which molten solder exerts on an electronic component is greater than the adhesive force exerted by the tackiness of the solder in an unmelted state. Accordingly, if solder paste at a first end of an electronic device melts while the solder paste at a second end of the device is still unmelted, the surface tension exerted by the molten solder at the first end exerts a downward force on the first end which may be strong enough to detach the second end of the electronic device from the printed circuit board and cause the second end to swing upwards about the first end. When an electronic device undergoes tombstoning, the second end of the electronic device is not electrically connected to the printed circuit board, so the device cannot function properly. Tombstoning is particularly a problem with leadless chip components, which are extremely small and commonly have maximum dimensions less than a millimeter, making them very susceptible to tombstoning when there is an imbalance in the forces acting on their ends during reflow soldering.
It has been discovered that tombstoning can be prevented by the use of a twin-peak solder alloy, which is an alloy which has two peaks in a DSC (differential scanning calorimeter) curve between its solidus and liquidus temperatures, with some phase transformation occurring at the temperature of each peak. In contrast, a eutectic solder alloy has only a single peak in a DSC curve between its solidus and liquidus temperatures.
However, twin-peak solder alloys which have been proposed thus far for use in solder pastes are lead-containing solders. For example, U.S. Pat. No. 6,050,480 discloses twin-peak solder alloys containing up to nearly 40 mass % of lead.
In recent years, there has been a movement in the electronics industry away from the use of lead-containing solders due to the environmental damage which can occur when electronic equipment employing lead-containing solders is discarded in landfills and lead in the solder leaches into the water supply.
Accordingly, there is a great need by the electronics industry for a lead-free solder paste which is suitable for reflow soldering of electronic devices without the occurrence of tombstoning.
The present invention provides a lead-free solder paste which can be used for reflow soldering without causing tombstoning.
The present invention also provides a method of reflow soldering using a lead-free solder paste.
According to one form of the present invention, a solder paste comprises a mixture of a powder of a lead-free Sn-based solder alloy and a flux mixed with the powder. The solder alloy is a twin-peak solder having two peaks in a DSC curve between its solidus and liquidus temperatures and contains 0.2-1.0 mass % of Ag. The first peak occurs at the start of melting of the alloy, and the second peak occurs at a higher temperature than the first peak when the major portion of the solder alloy subsequently melts. Preferably the magnitude of the first peak is less than or equal to the magnitude of the second peak.
The solder alloy may include various additional components, such as a strength improving element, a melting point lower element, or an oxidation preventing element.
According to another form of the present invention, a soldering method comprises performing reflow soldering of electronic devices to a printed circuit board using a solder paste according to the present invention.
The present invention is not restricted to use with any one type of electronic device, but it is particularly suitable for the reflow soldering of leadless chip components.